In a conventional automatic transmission, pressure levels of oil selectively supplied to plural frictional engagement elements (frictional clutches and frictional brakes) have been controlled, at times of engagement and disengagement of such elements, by means of a combination of an accumulator and an orifice. However, because, in order to perform the function of an accumulator to a sufficiently high degree, an accumulator having a relatively large volume is required, the volume of an automatic transmission provided with plural accumulators for purposes of attaining a sufficient degree of oil pressure, may rise to an undesirable level.
In light of the foregoing, proposals have recently been made to introduce a method of controlling pressure levels of oil supplied to these frictional engagement elements without employing an accumulator. Specifically, in this oil pressure level controlling method, pressure levels of oil supplied to the frictional engagement elements are controlled by controlling directly, by means of a solenoid valve, a pressure level of oil from an oil pressure source. So far, a smooth and highly positive response has been achieved at a shift operation.
As an oil pressure control apparatus for this type of automatic transmission, JP06(1994)-341525A discloses an oil pressure control apparatus for an automatic transmission linked to an engine, an automatic transmission in which a shift stage is established by releasing (disengaging) a predetermined, first, frictional engagement element and by engaging another, second, frictional engagement element. The oil pressure control apparatus includes: a pressure regulating mechanism capable of regulating a pressure level of oil supplied to the second frictional engagement element, and at the same time capable of adjusting a pressure regulating property; a pressure regulation commanding means which outputs a signal of command to the pressure regulating mechanism; shift condition detecting means for detecting at least either rise in engine fuel at a shift operation or engagement and disengagement of the frictional engagement elements; and pressure regulating property changing means for changing, on the basis of a detecting result of the shift condition detecting means, a pressure regulating property of the pressure regulating mechanism.
In this oil pressure control apparatus for an automatic transmission, the level of the oil pressure is selected, in accordance with a shift line diagram (a map or a database), from among whether it is a level of a line pressure, whether the level of the oil pressure has dropped, and whether the level of the oil pressure has risen. When a shift operation is performed in accordance with the shift line diagram, a shift shock can be absorbed by regulating a pressure level of oil supplied to a frictional engagement element, for example by reducing, or alternatively raising, a pressure level of oil supplied thereto.
When a disengagement condition, in which all frictional engagement elements in an automatic transmission have been disengaged, is switched to an engagement condition, in which a predetermined frictional engagement element, from among the frictional engagement elements, is engaged, i.e., when a neutral shift stage is switched to a shift stage appropriate to an actual vehicle driving condition, it is necessary that at least two of the frictional engagement elements be engaged. In recent years, oil pressure control elements such as solenoid valves have been reduced to the maximum possible extent, and the number of pressure regulating valves has accordingly been restricted to a minimum. As a result, at least one of the two frictional engagement elements is being introduced with oil with line pressure not being regulated. If oil with line pressure is supplied to one of the two frictional engagement elements, and oil pressure increasing control, or oil pressure decreasing control, is applied to the other, this makes it impossible to control the pressure level of oil with line pressure exerted on the one of the frictional engagement elements. In such circumstances, when a high degree of line pressure from an oil pump is exerted on the first frictional engagement element, an engagement condition is suddenly produced, synchronization of rotation of an input-side rotational member of the frictional engagement element and rotation of an output-side rotational member thereof occurs abruptly, and on occasions this may result in a high degree of shift shock.
The present invention has been made in view of the above circumstances, and provides an oil pressure control apparatus for an automatic transmission capable of avoiding the occurrence of shift shock, at the time of a shift operation from a neutral shift stage to an other shift stage in an automatic transmission while a vehicle is being driven, even when oil with line pressure is supplied to a frictional engagement element.